gadgets.io.getProxyUrl() is supposed to be used to cache static content on NK servers. Given that, it cannot be signed. If you need to request dynamic reources or sign the request (which, by definition, makes it non-cacheable) the you must use gadgets.io.makeRequest()

Another thing is that I really do not see any reason that game should load an iframe proxied through NK servers. You can just load it directly (you won't get direct access to NK JS API inside that iframe, but that would be the same for gadgets.io.getProxyUrl()).